Hybrid Photoresist Helps Meet Demand for Smaller Chips

To achieve performance gains in microelectronics, semiconductor size continues to shrink, with feature sizes now in the sub-10-nm range. The challenges associated with the extreme downscaling of semiconductors have led to the adoption of a powerful semiconductor fabrication method: extreme-ultraviolet (EUV) lithography.

EUV lithography requires photoresists that provide high sensitivity, resolution, and etch selectivity. To meet this need, researchers at the Center for Functional Nanomaterials (CFN), a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory, designed a light-sensitive, organic-inorganic hybrid EUV photoresist to pattern small semiconductor features for high-volume manufacturing.

The positive-tone, organic-inorganic hybrid EUV photoresist provides high-resolution EUV lithography and electron-beam lithography patterning, combined with high sensitivity and etch resistance. Further, the low-exposure doses required to perform high-resolution patterning in the hybrid resist, along with the high etch resistance and the team’s well-developed resist processing strategy, could lead to the use of infiltration-synthesized, hybrid thin films as reliable EUV lithography photoresists that support the development of next-generation semiconductor devices and microelectronics.

The researchers made the resist from poly(methyl methacrylate) infiltrated with indium oxide (PMMA-InOx). It is synthesized via vapor-phase infiltration, a material hybridization technique derived from atomic layer deposition. The PMMA-InOx hybrid resist leverages high EUV-absorbing and etch-resistant inorganic species, by using InOx. It also exploits the weak interaction of organometallic indium precursor with the PMMA matrix to achieve the uniform distribution of infiltrated InOx in PMMA during VPI, thus ensuring good patterning performance.

“To synthesize our new hybrid resist materials, organic polymer materials are infused with inorganic metal oxides by a specialized technique known as vapor-phase infiltration,” said researcher Chang-Yong Nam, who led the project. “Compared to conventional chemical synthesis, we can readily generate various compositions of hybrid materials and control their material properties by infusing gaseous inorganic precursors into a solid organic matrix.”

The weak binding of the gaseous indium precursor to the carbonyl group in PMMA allows the synthesis of hybrids with inorganic content to be distributed uniformly in the resist, enabling high-sensitivity EUV and EB lithography. The hybrid resist demonstrated high-resolution, positive-tone EUV lithography patterning, with a 40-nm half-pitch line-space and about 50-nm-diameter contact hole patterns with reasonable uniformity in critical dimension. The high silicon (Si) etch selectivity of the hybrid resist enabled the generation of high-aspect-ratio Si nanostructures via plasma etching pattern transfer to the underlying Si substrate.

In earlier research, the team worked with an established resist composition as a proof of concept.

“In this new paper, we used a composition that hasn’t been studied in the resist community, yielding better EUV absorption and improved patterning performance,” said researcher Nikhil Tiwale.

In this artist’s rendition, mirrors focus extreme-ultraviolet (EUV) light to pattern a latent image in a polymer thin film infiltrated by indium-containing gaseous molecules. Courtesy of Brookhaven National Laboratory.

The hybrid materials have an increased sensitivity to EUV light, so they do not need to be exposed to as much EUV light during patterning, which reduces process time. The new materials also show improved mechanical and chemical resistance, making them more suitable to use as templates for high-resolution etching.

When the team initially made the hybrids, one of the challenges it encountered was how to ensure the uniform distribution of the inorganic content inside the organic polymer, while also ensuring that the infused inorganic components were not bound to the organic matrix too strongly. According to researcher Ashwanth Subramanian, the team selected a different precursor for the metal — or the inorganic source.

The resulting hybrid featured a uniform composition and weak binding between the organic and inorganic components. The use of indium, instead of aluminum, as an inorganic component increased the sensitivity of the material and provided a more uniform material makeup, the researchers specified.

The team is developing and testing other hybrid material compositions and is also working on the processes involved in fabricating the materials, paving the way for patterning smaller, more efficient semiconductor devices.

Nam will participate in the DOE Accelerate Innovations in Emerging Technologies program, a multi-institute project that will explore the development of new classes of hybrid photoresists and use machine learning to accelerate EUV research by making material validation easier and more accessible.

“It’s currently really hard to do EUV patterning,” he said. “The actual patterning machine that industry is using is very, very expensive — the current version is more than $200 million per unit! There are only three to four companies in the world that can use it for actual chip manufacturing.

“There are a lot of researchers who want to study and develop new photoresist materials but can’t perform EUV patterning to evaluate them. This is one of the key challenges we hope to address.”

The research was published in Advanced Materials Interfaces (www.doi.org/10.1002/admi.202300420).